First ask yourself what IS purchasing power? What units would you use to even quantify such a thing?

Next think about what CPI is and what is the consequence of prices increasing per dollar... how is this related to purchasing power as you defined it above. Can you quantify a change in purchasing power per dollar (as a ratio)?

Finally, given your conclusions thus far regarding purchasing power per dollar... what does the increase in wage mean (proportionally speaking)?

So, in conclusion, what is the Purchasing power of the worker in 2017 versus 2011 given both changes in CPI and wage (expressed as a ratio)?